1. Technical Field
The present invention relates to a motor control apparatus for controlling a synchronous motor such as brushless motor and reluctance motor by using an electric power converter such as inverter, and more particularly, to a motor control apparatus for controlling the speed, etc. of a synchronous motor, without using a sensor for detecting the motor axis position.
2. Related Art
There are a rectangular drive system (120 degree excitation system) and a sinusoidal drive system (180 degree excitation system) that serve as a drive system for brushless motor that is well known as synchronous motor. The sinusoidal drive system supplies a sinusoidal phase current to a motor, to realize low vibration, low ambient noise, and high efficiency, and is superior in these respects to the rectangular drive system.
As a motor axis detecting system for brushless motor, there are one that utilizes a signal from a position detecting sensor such as rotary encoder and resolver, and one that acquires position information from the phase current and phase voltage of a motor, without using a position detecting sensor. The latter or sensorless system does not require the provision of a position detecting sensor and wiring therefor, to thereby achieve a reduction in cost, and is superior in this respect to the former sensor system with sensor.
Under these circumstances, attempts have recently been made to control a brushless motor by means of a sensorless sinusoidal drive system (see for instance, JP-A-2001-268974, JP-A-2001-251889, JP-A-2000-350489, “Sensorless salient type brushless DC motor control based on speed electromotive force estimation,” Takeshita, Ichikawa, Li, Matsui in Nagoya Industry University, T. IEE Japan, Vol. 117-D, No. 1, 1997). An example of sensorless sinusoidal drive systems is shown in FIG. 1.
In FIG. 1, reference numeral 51 denotes a motor; 52, electric power converting section (inverter); 53, DC power supply; 54, PWM signal generating section; 55, phase voltage determining section; 56, applied current determining section; 57, rotational speed command; 58, current/voltage detecting section; 59, d/p-axis converting section; 60, axis position predicting section; 61, motor rotational speed calculating section; and 62, 63, adders.
The applied current determining section 56 determines an applied current based on the rotational speed command 57 and motor rotational speed, and the phase voltage determining section 55 determines a phase voltage based on a predicted axis position and current phase. The current/voltage detecting section 58 detects a phase current and phase voltage to the motor, the d/p-axis converting section 59 performs a d/p-axis conversion of the phase current and phase voltage from the current/voltage detecting section 58, based on current phase information from the axis position predicting section 60, and the axis position predicting section 60 predicts, by calculation, an axis position of the motor 51 based on d- and p-axis components Id, Ip, Vd, Vp of current and voltage and the motor rotational speed. The motor rotational speed calculating section 61 calculates an angular velocity ω on the basis of current phase information supplied from the axis position predicting section 60, etc.
The foregoing motor control apparatus controls the motor operation by means of a control loop for determining the phase voltage to the motor so as to maintain the phase current to the motor at constant, a control loop for predicting the motor axis position from the applied phase current and phase voltage, and a control loop (motor speed feedback loop) for calculating the motor rotational speed from the predicted motor axis position and for changing the constant phase current to the motor. This motor control apparatus entails a drawback that complicated calculations are required to control drive of the motor since the double feedback control for current is inevitably necessary, and a drawback that a control circuit which is high in processing speed and hence high-priced is required since both the current feedback control and the speed feedback control must be made at extremely high speeds. It entails a further drawback that, in addition to complicated calculations being required for the prediction of motor axis position, a sufficiently high position accuracy cannot be attained even by such complicated calculations being made.